BUILD: http: silence an uninitialized warning affecting gcc-5

BUILD: http: silence an uninitialized warning affecting gcc-5

When building with gcc-5, one can see this warning:

  src/http_fetch.c: In function 'smp_fetch_meth':
  src/http_fetch.c:356:6: warning: 'htx' may be used uninitialized in this function [-Wmaybe-uninitialized]
     sl = http_get_stline(htx);
        ^

It's wrong since the only way to reach this code is to have met the
same condition a few lines before and initialized the htx variable.
The reason in fact is that the same test happens on different variables
of distinct types, so the compiler possibly doesn't know that the
condition is the same. Newer gcc versions do not have this problem.
Let's just move the assignment earlier and have the exact same test,
as it's sufficient to shut this up. This may have to be backported
to 2.6 since the code is the same there.